1. Field of the Invention
The invention relates to methods for manufacturing hollow components made of laminated composite materials, particularly laminated composite materials comprising reinforcing fiber layers embedded in a hot-polymerized resin. More specifically, it relates to manufacturing methods using cores to form the components' cavities.
The invention also relates to methods for manufacturing such hollow components having walls enclosing the cavities which are adjacent to non-enclosing walls, which do not subtend the cavities.
2. Description of the Related Art
Laminated composite components comprising reinforcing fiber layers embedded in a hot-polymerizing resin are used, in particular, in the automobile, space and aeronautic industries because of their excellent strength-to-weight ratio. In general, the reinforcing fibers are carbon fibers or silicon carbide fibers and the resins are epoxides, bismaleimides or poly-imides. In particular, one seeks to manufacture components such as reservoirs, ducts, air manifolds etc. with the demands of material quality, surface condition, mechanical and thermal strength, and high dimensional accuracy. These components comprise thin enclosing walls; that is, walls enclosing and subtending cavities, which may possibly communicate with the outside through minute apertures. Such components are desired to have the enclosing walls adjacent to non-enclosing walls (ones not subtending cavities). Illustratively, they may be a casing comprising an air manifold at its surface.
A well known procedure for making hollow components composed of fiber/resin laminated composites uses a mold matching the outer shape of the component and generally being at least in two parts for the purpose of removing the component. This known procedure in particular includes the following essential operations:
making a core in the shape of the component's cavity; PA1 arranging an inflatable balloon around the core; PA1 cladding the core; that is, placing the resin-preimpregnated reinforcing fibers constituting the composite around the combination of the core and the balloon; PA1 arranging the combination of the core, the balloon and the composite in the mold; PA1 hot-polymerizing the resin with the balloon being pressurized; PA1 hot removal of and cooling of the component; PA1 withdrawing the core; and PA1 withdrawing the balloon through the cavity aperture. PA1 1) the inside surfaces are rough and fairly inaccurate because the pressurized balloon follows the deviations in the cladding; furthermore, the balloon poorly pushes the composite into the angular recesses such that the component consequently remains imprecise at those sites; PA1 2) removal of the balloon after molding may be difficult when the cavity is large and the aperture small, thus constraining the designer to provide adequate aperture dimensions; PA1 3) the balloon must be kept in perfect condition; that is, it must remain sealed and its flexibility must be preserved as long as the resin is above its solid-state polymerizing temperature; however, some resins, such as the polyimides which are kept at high temperatures, have polymerization melting points higher than 300.degree. C. The inventor is unaware of any elastomers with which to make balloons withstanding such temperatures; consequently, the procedure cannot be carried out using such resins. PA1 cladding; that is, applying the resin-impregnated fiber layers against the mold to form the composite; PA1 covering the composite with a film which is impermeable relative to the mold, the expert previously having arranged a conduit connected to a partial-vacuum source as well as different fabrics which promote evacuation and removal (such techniques are mentioned merely for elucidation but are unrelated to the implementation of the invention); PA1 hot, autoclave polymerization at the required temperature and pressure, the pressure in the autoclave forcing the composite, by the film against the mold surface, to compact this composite and to shape it as required. PA1 a) manufacturing a core of a shape which corresponds to that of the cavity to be made; PA1 b) cladding the core with at least one layer of resin-preimpregnated fibers to form a fiber composite; PA1 c) arranging the core clad in the composite in the mold; and PA1 d) hot-polymerizing the resin. PA1 manufacture of the core's body; PA1 arranging the body in the mold, spacers made of crosslinked, that is polymerized, silicone elastomer being placed between the body and the mold's walls, each spacer being of the thickness of the silicone elastomer layer required at that location; PA1 pouring liquid silicone elastomer into the space left free between the core and the mold's wall; and PA1 cold crosslinking the silicone elastomer followed by removal from the mold. PA1 a) manufacturing a core to be shaped like the component's cavities; PA1 b) cladding the mold, that is the shaping surfaces and the mold-cavities, with at least one layer of resin-preimpregnated fibers to constitute a first composite layer; PA1 c) arranging the core in the mold-cavities above the first composite layer; PA1 d) cladding the above assembly with at least one second layer of resin-preimpregnated fibers to constitute a second composite layer, the core thus being trapped between the two composite layers; PA1 covering the combination of the composite and the core with a film to seal it with respect to the mold; PA1 compression and hot-polymerization in an autoclave, wherein, during resin polymerization, the core applies an outward compression caused by thermal expansion to the composite located on the side surfaces of the mold forms, and wherein the core comprises a body covered by a layer of silicone elastomer having a high thermal coefficient of expansion a and an appropriate thickness e.
The core's melting point may be lower than the onset temperature of resin-polymerization, for instance paraffin or wax. Following the enclosing process, the core may be withdrawn in molten form. Moreover, resin cores are known which dissolve in an organic solvent; thus, after component removal and cooling, the core may be removed in dissolved form from the cavity. However, this procedure has the drawback of degrading the resin of the component, hence the component's surface quality and its strength. Moreover, water-soluble ceramic cores are known. This latter approach appears to be the most appropriate one for the case under consideration. Pressurizing the balloon allows pushing and compressing the composite against the wall of the mold to reduce the inherent swelling of the preimpregnated fiber layers. The expression "swelling" herein denotes the additional thickness of the preimpregnated fiber layers before compression. In this manner, a homogeneous material is produced free of micro-cavities between the fibers. Typical pressures run between 8 and 15 bars, the thickness of the composite thus being reduced by between 15 and 30%. In other words, a density increase, i.e. compaction, is involved.
Nevertheless, this technique has the following intrinsic drawbacks:
The components consisting of thin and non-enclosing walls (walls not forming cavities) are conventionally manufactured by a so-called "bag" molding technique using an open mold in the shape of the wall, more specifically in the shape of one of the two surfaces of the wall. This procedure in particular comprises the following stages:
However the bag-molding technique cannot be combined with the balloon-molding technique to manufacture an integral component comprising at least one first hollow part (derived by the balloon molding technique) and at least one thin wall not forming a cavity (derived by the bag molding technique). At least one portion of the enclosing wall subtending the cavity will simultaneously be between the bag and the balloon, that is between two pressurized flexible membranes. That wall portion will assume a significantly random shape even when there is rigorous equilibrium between the pressure exerted on the bag and that on the balloon, for instance, when bag and balloon are made to communicate. Consequently, the geometry of a component manufactured by such a combined technique will always be unreliable.